This invention relates to apparatus for detecting discontinuities, such as air bubbles, in a fluid flow utilizing ultrasonic energy.
Various arrangements have been proposed for detecting bubbles, discontinuities, such as air particles, solids in a flow of liquid, particularly where the liquid is in a tubing which is either rigid or compressible. Some of the uses for such a discontinuity detector would be, for example, in detecting air bubbles in body fluids, such as blood, which are being transmitted from one place to another either with the patient in the transmittal loop or from one type of a machine, such as blood processing machine, to another such machine.
Various arrangements previously have been provided for such detection. Included among these are, for example, U.S. Pat. No. 3,921,622 to Cole in which bubble detection is accomplished by detecting a change in amplitude of a received ultrasonic pulse versus the amplitude of a pulse which passes through air. Another patent is U.S. Pat. No. 3,974,681 to Namery which uses an amplitude measuring technique, with the components being optimized at particular resonant frequencies of the system.
In the system disclosed in U.S. Pat. No. 4,122,713, to Stasz, a doppler and backscatter technique is used. In Liebermann U.S. Pat. No. 4,138,879, bubbles are detected by transmitting ultrasonic signals through a tube from one transducer to another with an amplifier being maintained in a marginally oscillatory condition. The detection of the bubbles changes the operating state of the system, and the change of state is detected.
In Bilstad U.S. Pat. No. 4,341,116 an arrangement is used in which amplitude levels are detected by a comparator. Liebermann U.S. Pat. No. 4,392,374 uses an adjustable bandpass technique while Abts U.S. Pat. No. 4,237,720 utilizes an ultrasonic transducer as a focusing lens. Another patent of interest is U.S. Pat. No. 4,068,521 to Cosentino et al, in which both continuous wave and pulses of ultrasonic energy can be used. The reception or non-reception of the energy is determined on an amplitude basis.
Some of the aforementioned bubble detectors, particularly those detecting on the basis of amplitude of received signal, have a problem in detecting the bubbles due to the size of the tube in which the fluid flows, the aging of the tube, which reduces its wall thickness and its flexibility, tube wall thickness and also with respect to bubble size. All of these problems give rise to variations in amplitude of the detected received signal. For a constant gain and constant threshold circuit, reliable detection of an air bubble becomes a difficult problem.